Method of heat treating age-hardenable alloys



United States Patent 3,390,023 METHOD OF HEAT TREATING AGE-HARDENABLE ALLOYS Chester S. Shira, Canoga Park, CaliL, assignor to North American Rockwell Corporation, a corporation of Delaware No Drawing. Filed Feb. 4, 1965, Ser. No. 430,496 6 Claims. (Cl. 148-127) ABSTRACT OF THE DISCLOSURE A method of heat treating age-hardenable nickel chromium base alloys which comprises controlling the temperature and rate of cooling of the alloys after being solution heated to produce annealed alloy bodies having a yield strength of at least 90,000 psi.

This invention relates to a method of heat treating agehardenable nickel-chromium base alloys. The invention was made in the performance of work under a NASA contract and is subject to the provisions of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 426; 42 USC. 2451), as amended.

The alloys to be heat treated according to this invention are hardenable through precipitation of minute particles of an intermetallic complex different from the matrix of base metal of the alloy. The metal ingredients of the complex are dissolved or dispersed throughout the base metal as a solid solution at an elevated temperature, known as the solution temperature, which is well under the melting temperature of the alloy. The particles of the complex are often submicroscopic, they being present in greater concentrations along the crystallographic slip planes of the matrix of the alloy mass, whereby they oppose or restrict relative movement along these planes, thus reducing the plasticity and increasing the hardness and strength of the alloy.

The alloys of this invention are rich in nickel, containing from about 40 to about 80 weight percent of nickel and from about 10 to about 25 weight percent of chromium. Their quality of hardening upon aging through precipitation of an intermetallic complex is derived from their content of aluminum and titanium, with optional partial substitution of either or both of the aluminum and titanium with columbium, the combined content of aluminum, titanium, and possibly columbium being from 2.6 to about 8.0 weight percent. The chemical formula of the precipitate complex is understood to be Ni AlTi or.

Ni AlTiCb when columbium is present as a third hardening element.

When an alloy of this invention is heated to or above its solution temperature and is then allowed to cool slowly the particles of the intermetallic precipitate become relatively large in size and relatively small in number, with the result that their concentrations along the crystallographic slip planes of the matrix are relatively low. Consequently, slowly cooled age-hardenable alloys are relatively low in hardness and strength. The customary heat treatment for these alloys involves solution heating followed by quenching in water whereby reprecipitation and growth of the particles of the hardening complex is initially prevented. Immediately upon being so quenched, the alloy is a super saturated solid solution which does not remain stable at room temperature, but instead, over a period of time, i.e. aging, gradually assumes its normal structure of distinct matrix and precipitate compositions. When so aged, the particles of the intermetallic precipitate are relatively small in size and relatively large in number whereby a high degree of hardness and strength results.

The particular age-hardenable alloys to which this in- 3,390,023 Patented June 25, 1968 vention pertains, they being of high nickel content, are especially well suited for use under conditions of relatively high temperatures, i.e., within the range of from about 1200 F. to about 1900 F. They have been used, for example, to form rocket thrust chambers which have been designed with a requirement for minimum yield strength of 90,000 psi. This strength requirement is met by the high temperature alloys of this invention which after being solution heated and water quenched are aged at 1300 F. for sixteen hours. Rocket thrust chambers are designed to meet their functional and eificiency requirements through their walls being of varying cross-sectional dimensions. For chambers of large size the thickness of the chamber walls and their varying degrees of thicknesses are too great to permit sudden quenching without risking substantial cracking and warping of the chamber. Also, in the manufacture of rocket thrust chambers of large size various components are brazed to the high temperature alloy bodies of the chambers before the chambers are to be subjected to an annealing heat treatment. For large size rocket chamber assemblies it is impractical to obtain optimum size and number of precipitation particles through the method of super saturation by water quenching and subsequent aging and still prevent damage to the brazed joints and other components of the assemblies.

This invention provides an improved method of heat treating age-hardenable nickel-chromium alloys for hightemperature use. The method comprises controlling the temperature and rate of cooling of the alloys after being solution heated to produce annealed alloy masses having a yield strength of at least 90,000 p.s.i. The method is simple, convenient, and especially expeditious. It provides an optimum concord of size and number of the precipitate particles for effectively opposing relative movement along the crystallographic slip planes of the matrix of the alloy mass to an extent that will result in a yield strength of at least that of the above identified minimum requirement. More particularly, the method of this invention involves the steps of solution heating the alloy at a temperature and for a time period sufficient to form a solid solution in which the elements of the precipitate are substantially uniformly dispersed throughout the alloy mass; thereafter cooling the mass at a rate of from about 110 F. to 20 F. per minute, preferably about 15 per minute, to an aging temperature of from about 1300 F. to about 1450 F.; maintaining the alloy mass at said aging temperature for a period of from about 10 to minutes, preferably from about 30 minutes to one hour; thereafter cooling the alloy mass at a rate of from about 10 to 20 F. per minute, preferably about 15 per minute, until a temperature of from about 900 to 1100 F., preferably about 1000 F., is reached; and thereafter allowing the alloy mass to cool to room temperature at an uncontrolled rate.

For the purpose of more specifically designating the particular high temperature type of age-hardenable nickelchromium alloys to be heat treated according to this invention, examples of these alloys are listed hereinafter in Table I with the percent composition by weight of those elements which characterize the respective alloys being correspondingly entered in the table. In the industry of manufacturing articles from these alloys, it is recognized that practically any generally recognized species of such alloys though marketed under a trademark may have been compounded by its manufacturer as for special purposes with any of various and slightly different contents of its respective ingredients. Accordingly, it is the usual practice in industry to identify the sundry trademark species of the various alloys with values commonly called nominal values of those metal ingredients which will readily distinguish one trademark group from another without listing the various impurities thereof, e.g. magnesium,

silicon, sulphur, phosphorous, etc. which do not materially affect the basic characteristics of the various alloys. Accordingly, in the following Table I, the values listed therein for representing the percentage composition by weight of the respective ingredients for the corresponding alloys listed in column 1 of the table are such nominal values, which do not add up to 100 percent, it being understood that the particular elements listed of the table are those which will characterize the respective alloys 4 of from about 1450 F. to about 1800 1 within thirty minutes time the precipitate particles become relatively large and sparse, they being resolvable with optical microscopy under magnification of 1500 times, and the yield strength of the alloy falls within the range of from below 90,000 p.s.i. to about 60,000 p.s.i. When the same alloy, after being solution heated, is aged at a temperature of from about 1000 F. to about 1300 F., its precipitate can barely be sensed with an electron microscope and its and the table does not include the percentage compositions yield strength may reach a value of over 130,000 p.s.i.;

of the several impurities which may be present.

however, the aging heat treatment must be maintained TABLE I Name of Alloy C Cr Al Fe Other Astroloy 15.00 4. B, .03. DCM 14. 30 4. 30 4. 60 B, .08.

10 15. 3.00 10.00 B, .04. Unitemp R235.... .10 15. 50 2. 25 7.00 Inconel W .04 15. 00 0. 7. 00

.05 18. 00 0. 00 12.00 Nicrotung 10 12. 00 4. 00 B, 058; gr, 02,

Nimonic .04 21. 00 0. 60 Nimonic 80A 10 20. 50 1. 25 Nimonic 00 10 19. 50 1. 20 Nirnonic 15.. 10 20. 00 2. 00 Nimonic 100. 10 11. 00 5.00 Nimonic 105. 10 15. 00 5. 00

Udlniet 700.. 12 15.00 4. 25 .08. Unitemp 4l 00 19. 00 l. 05 .005. Unitemp M 25 15 10. 00 1.00 .005. Unitemp 500""... .12 17. 50 3.00 .005; Zr, .05. Waspaloy .05 19. 50 25 1. 30 .005; Zr, .00.

For the purpose of generically designating the particular class of high-temperature alloys to be heat treated according to this invention, they may be identified as the balance being nickel and impurities, the nickel being present to at least 42 percent, and the combined content of aluminum, titanium, and columbium being in the range of 2.6-8.0 percent.

The minimum temperature, i.e. solution temperature, to which any of these age-hardenable alloys is to be heated to form a homogeneous solid solution as distinguished from precipitate and matrix phases, lies within the range of from about 1725-2250 F. The time-period for such solution heating varies depending upon the thickness of the alloy mass. For example, a mass of about one-eighth inch thick requires a solution heating period of about 10 minutes, while a mass of one and one-half inches thick requires a solution heating period of about one and onehalf hours.

As mentioned hereinabove, slow cooling of age-hardenable alloys, after being solution heated, exhibit intermetallic precipitate particles which are relatively large in size and small in number, whereby the alloy is relatively low in hardness and strength. When an age-hardenable alloy of the type hereinabove specified, after being solution heated, is air cooled to an aging temperature in the range in this range to nearly a full weeks time in order to eventually attain such strength. Again, when the same alloy, after being solution heated, is aged according to the schedule of this invention, i.e. from about 1300 F. to about 1450 F. for a period of time of from about 10 minutes to about minutes, its precipitate can be resolved with an electron microscope at a magnification of 20,000 times, and its yield strength ranges from 90,000 p.s.i. to about 110,000 p.s.i. Thus, it is that the heat treatment of this invention provides a simple and expeditious method for hardening age-hardenable alloys of the type herein specified to a yield strength of at least 90,000 p.s.i.

When the strength properties, e.g. yield strength, of an age-hardenable alloy are to be determined from the plotting of a curve on a graph of stress in p.s.i. vs. strain in percent elongation, it is found that such curve does not include the usual sharp change in direction, i.e. yield point, which is characteristic of the non-age-hardenable alloys. The strength curve for an age-hardenable alloy includes an initial straight line portion extending up to a point commonly called the proportional limit and such straight line portion corresponds in kind to that for a non-age-hardenable alloy. Beyond the upper end of the straight line portion for an age-hardenable alloy the strength curve gradually increases in rate of units of elongation per unit of stress until the point of breaking, i.e. ultimate tensile strength, is reached. Inasmuch as there is no definite point on the strength curve for an age hardenable alloy which corresponds to the distinct yield point on the stress curve for a non-age-hardenable alloy, it is conventional practice to draw a line parallel to the initial straight line portion of the strength curve for the age-hardenable alloy spaced at a distance representing a two-tenths percent increase in elongation. The value in p.s.i. for the point of intersection of such a two-tenths percent elongation line and the strength curve is known as the yield strength of an age-hardenable alloy and the term yield strength as used herein has that meaning.

The invention is hereinafter illustrated in greater detail by description of the following specific examples of the practice of it:

Example I In the preliminary stages of the manufacture of an F-1 rocket engine, its thrust chamber parts are assembled, brazed, and annealed. The inside and outside surfaces of such thrust chamber are defined by a circumferentially arranged series of contiguous tubes extending in an axial direction, each tube being brazed to its next adjacent tube and to a series of axially spaced encircling restricting bands. The tubes and bands are set in place around a mandrel, a combustion zone jacket is mounted at the forward ends of the tubes, and forward and aft end rings are set in place, whereupon the assembly is treated with brazing composition along all abutting surfaces for joining the parts together. The assembly is then placed in a furnace to effect brazing. For the F-1 thrust chamber, its tubes, restraint bands, and combustion zone jacket are formed of Inconel X.

In this example of the practice of the invention, the brazing and solution heating operations for an F-l thrust chamber assembly were performed as a single heating step, i.e. the assembly of the mandrel-mounted tubes, restraint bands, combustion zone jacket and end rings, after application of brazing composition, was placed in a furnace having an inert atmosphere of the group consisting of nitrogen, argon, and hydrogen. Several test bars formed of Inconel- X and of thicknesses equal respectively to the thicknesses of the tubes, restraint bands, and the walls of the combustion zone jacket were suspended in various parts of the furnace. The inside of the furnace was raised to a temperature of about 2100 F. and maintained at that temperature for a period of about minutes to effect brazing and solution heating. The temperature of the furnace was then decreased at the rate of about F. per minute to an aging or stall temperature of about 1325 F., and was maintained at that temperature for a period of about one hour. The temperature of the furnace was then decreased at the rate of about 15 F. per minute to a temperature of about 1000 F. and thereafter air cooled at an uncontrolled rate to room temperature. Prior to removal of the thrust chamber from the furnace the specimen test bars were subjected to tensile tests for determination of their mechanical properties and all of them were found to exhibit a yield strength of at least 90,000 p.s.i.

Example II For a I-2 rocket engine, a thrust chamber assembly of mandrel-mounted tubes and restraint bands, a combustion zone jacket, and end rings, after application of brazing composition, was placed in a furnace for a first brazing cycle. For the I-2 thrust chamber, the tubes which define the chamber walls and some of the restraint bands are formed of stainless steel, while others of the restraint bands and the combustion zone jacket are formed of the age-hardenable alloy, Inconel 718. Accompanying the thrust chamber assembly in the furnace was a plurality of representative test bars of Inconel 718. The furnace Was heated up to a temperature of about 2100 F. and maintained at that temperature for a period of about 10 minutes, and thereafter cooled to room temperature at a rate of about 10 F. per minute. The thrust chamber assembly was then turned upside down in the furnace, additional brazing alloy of a different composition than that for the first brazing cycle was applied throughout the contiguous parts of the assembly, and the assembly was heated to a brazing and solution heating temperature of 1850 F. for a period of ten minutes. Thereafter the temperature of the assembly was cooled at a rate of about 15 per minute to a stall temperature of about 1325 F. at which temperature it was maintained for a period of about one-half hour. Thereafter, the chamber was cooled at a rate of about 15 per minute until a temperature of about 1000 F. was reached whereupon the chamber was air cooled at an uncontrolled rate to room temperature. It was found that all of the test bars of Inconel 718 alloy exhibited a yield strength of at least 90,000 p.s.i.

The heat treatment schedule of this invention as it is described hereinabove includes a rate of cooling from the solution heating temperature to the aging or stall temperature, and from the aging temperature to preferably about 1000 F., whereupon the alloy may be cooled at an uncontrolled rate without materially affecting its strength. Such preferred cooling rates prevent buckling and warping of large sized bodies of the alloys.

It will be understood that it is intended to cover all changes and modifications of the examples of the invention herein chosen for the purposes of this disclosure which do not constitute departures from the spirit and scope of the invention.

Having described the invention, what is claimed is:

1. A method for heat treating an age-hardenable nickelchromium base alloy for high-temperature use, whereby the alloy exhibits a yield strength of at least 90,000 p.s.i., the method comprising the steps of solution heating the alloy to a homogeneous solid solution;

cooling the alloy from its solution heating temperature at a rate of from about 10 to 20 F. per minute to an aging temperature of from about 1300 F. to about 1450 F.;

maintaining the alloy at said aging temperature for a period of from about 10 to minutes;

cooling the alloy at the aforesaid cooling rate to a temperature of from about 900 to 1100 F.; and, thereafter cooling the alloy to room temperature.

2. A method of heat treat-ing an age-hardenable alloy to provide a yield strength of at least 90,000 p.s.i., the

alloy having the following percentage composition by weight:

Carbon 0.l.l5 Chromium 10-25 Tungsten Up to 8 Molybdenum Up to 10 Cobalt Up to 30 Columbium 'Up to 5 Titanium .65-4 Aluminum .3-5 Iron Up to .1 Boron Up to 20 Zirconium Up to .06

the balance being nickel and impurities, the nickel being present to at least 42 percent, and the combined content of aluminum, titanium, and columbium being in the range of 2.6-8.0 percent; the method comprising the steps of solution heating the alloy to a homogeneous solid solution;

cooling the alloy from its solution heating temperature at a rate of from about 10 to 20 F. per minute to an aging temperature of from about 1300 to 1450 maintaining the alloy at said aging temperature for a period from about 10 to 90 minutes;

cooling the alloy at the aforesaid cooling rate to a temperature of from about 900 to 1100 F.; and, thereafter cooling the alloy to room temperature.

3. The method of claim 2 wherein the first and second cooling steps are preferred at a rate of about 15 F. per minute.

4. The method of claim 2 wherein the step of maintaining the alloy at said aging temperature is conducted for a period of from about thirty minutes to one hour.

5. A method for brazing and heat treating an assembly of metal parts, comprising the steps of mounting the several parts of the assembly in contiguous relationship with each other upon a support, at least some of said 7' parts being formed of an age-hardenable alloy having the following percentage composition by weight:

the balance being nickel and impurities, the nickel being present to at least 42 percent, and the combined content of aluminum, titanium, and columbium being in the range of 2. 68.0 percent;

applying brazing composition to the contiguous surfaces of said mounted parts; heating the mounted parts in a furnace to a temperature within the range of about 1725-2250 F. for a period of time within the range of 10-90 minutes to effect brazing of the parts and solution heating of the age-hardenable alloy parts; cooling the brazed assembly at a rate of from about 1020 F. per minute to an aging temperature of from about 1300-1450 F.; maintaining the brazed assembly at said aging temperature for a period of from about 10-90 minutes; cooling the brazed assembly at the aforesaid cooling rate to a temperature of from about 9 1100 F.; and,

thereafter cooling the brazed assembly to room temperature. 1' i I 6. The method of claim 5 wherein the said age-hardenable alloy has the following percentage composition by Carbon Chromium 15-1 8 Molybdenum Up to 3.0 Columbium 1.0-5.0 Titanium .82.4 Aluminum .4-.-6 Iron 7-18 the balance being nickel and impurities, the nickel being present to at least 53 percent and the combined content of aluminum, titanium, and columbium being in the range of 3.0-6.2 percent; said brazing and solution heating being conducted at a temperature of from 1850-2100 F. for a period of about 10 minutes, and said aging temperature being about 1325 F. for a period of from one-half to one hour.

References Cited UNITED STATES PATENTS 3,145,124 8/1964 Hignett et a1 1481'62 3,207,599 9/1965 Franklin et a1. 148-462 X FOREIGN PATENTS 583,845 1/ 1947 Great Britain. 715,140 8/1954 Great Britain.

CHARLES N. LOVELL, Primary Examiner. 

